1. Field of the Invention
The present invention relates to an optical beam scanning system for securing a photosensitive material with plural optical beams to form an image on the photosensitive material, and more particularly, the present invention relates to a technique for independently changing a diameter of beam spots and a pitch of pixels on the photosensitive material.
The present invention further relates to an optical beam deflector, which is often used in an optical beam scanning system, comprising an acoustooptic deflector (or AOD), and more particularly, the present invention relates to an optical beam deflector which can compensate a cylindrical lensing effect of the AOD.
2. Description of the Prior Art
An optical beam scanning system using plural laser beams (or a multibeam scanning system) is often included in a laser plotter for recording a black-and-white image on a photosensitive material or in a scanner for recording a halftone image with halftone dots on photosensitive material, in order to reduce scanning time.
FIG. 1 is a diagram showing scanning lines drawn with a multibeam scanning system. Plural sensing lines L.sub.1 -L.sub.11 extending in a main scanning direction X are arrayed in a subscanning direction Y. For example, two beams simultaneously run along the scanning lines to form an image. The multibeam scanning system is adjusted so that two beam spots SP.sub.1 and SP.sub.2, which have a diameter d and which are separated from each other by a distance l, are formed on photosensitive material. The distance l satisfies the following equation: EQU l=(2n-1)P (1)
where P is a pitch of pixels (or a pitch of scanning lines), and n is a natural number. The natural number n is two in FIG. 1.
As shown with a pair of arrows AR.sub.1, the two beam spots SP.sub.1, and SP.sub.2 firstly run along the scanning lines L.sub.1 and L.sub.4. Secondly, the beam spots SP.sub.1 and SP.sub.2 are moved by a distance 2P relatively to the photosensitive material in a subscanning direction Y, thereby run along the scanning lines L.sub.3 and L.sub.6. The movement of the beam spots SP.sub.1 and SP.sub.2 in the subscanning direction Y and the scanning operation in the main scanning direction X are alternately repeated, whereby parallel scanning operation shown with pairs of arrows AR.sub.1 -AR.sub.4 are achieved. Accordingly the scanning lines L.sub.1, L.sub.3, L.sub.5, . . . , L.sub.11 specified with odd ordinal numbers are scanned with the first beam spot SP.sub.1 and the scanning lines L.sub.2, L.sub.4, . . . , L.sub.10 specified with even ordinal numbers are scanned with the second beam spot SP.sub.2.
The pitch of pixels is sometimes increased so that speed of forming an image is gained. On another occasion, the pitch of pixels is decreased so that an image is formed more minutely. When an optical beam scanning system using a single beam is used, the pitch of pixels can be adjusted through changing a clock pulse for controlling supply-timing of image data or through changing a scanning speed. When a multibeam scanning system is used, changing the pitch of pixels causes the following problem.
When a multibeam scanning system is used, the following equation holds: EQU (2n.sub.1 -1)P.sub.1 =(2n.sub.2 -1)P.sub.2 ( 2)
where P.sub.1 and P.sub.2 are pitches of pixels and n.sub.1 and n.sub.2 are natural numbers. If the natural numbers n.sub.1 and n.sub.2 satisfying the equation (2) are found, either of the pitches P.sub.1 and P.sub.2 is attainable regardless of a distance between the beam spots SP.sub.1 and SP.sub.2. On the contrary, if a pair of the natural numbers n.sub.1 and n.sub.2 satisfying the equation (2) are not found, the pitches P.sub.1 and P.sub.2 are not interchangeable unless the distance between the beam spots SP.sub.1 and SP.sub.2 is changes. This can also be explained as follows: A distance l.sub.1 between the beam spots SP.sub.1 and SP.sub.2 is given by the following equation when the pitch of pixels is P.sub.1 : EQU l.sub.1 =(2n.sub.1 -1)P.sub.1 ( 3)
On the other hand, if the pitch of pixels is P.sub.2, a distance l.sub.2 between the beam spots SP.sub.1 and SP.sub.2 is given by the following equation: EQU l.sub.2 =(2n.sub.2 -1)P.sub.2 ( 4)
Therefore, the distances l.sub.1 and l.sub.2 cannot be equal to each other unless the equation (2) is satisfied.
The distance l between the beam spots can be adjusted through changing a minification factor of a minifying optical system in the multibeam scanning system; this reduces the size of the beam spots formed on photosensitive material. However, the diameter d of the beam spots is also changed as well as the distance l by this method. The distance l, that is, the pitch of pixels cannot be independently changed by this method accordingly. Similarly, the diameter d cannot be changed separately from the pitch of pixels.
Meanwhile, the following problem is also known for a scanner which performs main scanning operation by deflecting at least one optical beam while performing subscanning operation by sequentially moving a beam scanning system relatively to the photosensitive material. The problem is that scanning lines are inclined due to the movement for the subscanning. FIGS. 2A through 2C illustrate inclined scanning lines formed on photosensitive material 1. An image-forming area 2 in the photosensitive material 1 is divided into a plurality of parallel strips 2a, 2b, . . . , 2z. The parallel strips 2a-2z are separately scanned in this order.
In FIG. 2A, an optical beam is cyclically deflected in a direction X while the photosensitive material 1 is moved in a direction (-Y), whereby the first strips 2a is scanned in a range between positions Y.sub.A and Y.sub.B in the direction Y. The other strips 2b-2z are scanned in the same manner. Since this divisional scanning method requires a smaller angle of deflection than a method in which a laser beam scans the full width of the image-forming area 2, the divisional scanning method causes less deflection errors. Further, if a width of scan is smaller, a focal length of a scanning lens can be reduced, and a diameter of a scanning beam is decreased, whereby an image can be more minutely formed. Moreover, the divisional scanning method can form an image faster than a method in which the main scanning and subscanning operations are performed by relatively moving a scanning optical system and the photosensitive material 1 mechanically because the divisional scanning method requires less movement of heavy members.
Even according to the divisional scanning method, scanning lines are inclined to the direction X in which a laser beam is deflected. The direction in which a beam spot runs on the photosensitive material 1 depends on a defection speed V.sub.X (not shown) and a subscanning speed (or a speed of moving the photosensitive material). The scanning line array 4 become to extend in a direction going up from left to right accordingly. As a result, an image formed on the photosensitive material 1 is also inclined to the direction X.
If all of the strips 2a-2z are scanned in a range between the positions Y.sub.A and Y.sub.B with a same deflection direction, the inclination can be compensated by several methods. According to one of the simplest methods, the inclination is compensated by setting an angle between the deflection direction and the direction in which the photosensitive material 1 is moved at a specific value deviated from 90 degrees. Japanese Patent Laying Open Gazette No.55-11917 also discloses a technique for compensating the inclination through deflecting a laser beam in the subscanning direction as well as the main scanning direction.
Incidentally, when the subscanning operation of all of the stripes are performed in a same direction, the photosensitive material 1 is fully returned in the direction Y after the scanning operation of each stripe is finished; this causes a time loss due to the returning movement. In order to eliminate the time loss, a reciprocating scanning apparatus is desired performing the subscanning operation from the position Y.sub.B to the position Y.sub.A in the stripes specified with even ordinal numbers.
When the reciprocating scanning operation is performed, the subscanning direction for the stripes specified with odd oridinal numbers (hereinafter referred to as odd-numbered stripes) is different from that for the stripes specified with even oridinal numbers (hereinafter referred to as even-numbered stripes). As shown in FIG. 2B, the inclination of the scanning line array 4 in the odd-numbered stripes is reversed from that in the even-numbered stripes. Consequently, if the above stated methods for compensating the inclination is applied, the inclination in the odd-numbered stripes might be compensated, but the inclination in the even-numbered stripes would increase as shown in FIG. 2C. Further, the degree of the inclination depends on the scanning speed V.sub.X and V.sub.Y.
As described above, the inclination of scanning lines is caused by various reasons. When the pitch of pixels and the diameter of beam spots are separately adjusted, it is important to compensate the inclination of scanning lines, as described later. However, the above stated methods of compensating the inclination are not suitable in this case. A new method of compensating the inclination is desired accordingly, when the pitch of pixels and the diameter of beam spots are separately adjusted.
In order to cope with the above stated problems, a first aspect of the present invention relates to an optical beam scanning system of a multibeam type which can adjust a pitch of pixels separately from a diameter of beam spots.
Incidentally, an optical beam scanning system often includes an acoustooptic deflector (or AOD) for deflecting optical beams to thereby perform the main scanning operation. A second aspect of the present invention relates to a device for deflecting an optical beam comprising an AOD.
Since the AOD can rapidly change an angle of deflection, it is often used in a system for performing high speed scanning of an optical beam.
When the AOD rapidly changes an angle of deflection, a so-called cylindrical lensing effect appears, as well known in the art. The cylindrical lensing effect is described in L. D. Dickson, "Optical Considerations for an Acoustooptic Deflector," Applied Optical, Vol. 11, No. 10, October 1972, pp.2196-2202.
FIG. 3A schematically illustrates the cylindrical lensing effect on an AOD. An AOD 213 comprises an acoustic cell 213a and a piezoelectric transducer 213b. An ultrasonic wave S generated by the transducer 213b propagates in the acoustic cell 213a. The ultrasonic wave S is schematically drawn with parallel lines in FIG. 3A. A smaller interval of the parallel lines means a higher frequency of the ultrasonic wave. The frequency of the ultrasonic wave S is linearly swept from a maximum value f.sub.max to a minimum value f.sub.min repeatedly.
When optical beams L.sub.a and L.sub.b are introduced in the acoustical cell 213a, diffracted beam L.sub.a1 and L.sub.b1 of a first order are produced. A diffracted angle .theta. of the diffracted beams L.sub.a1 and L.sub.b1 is given by the following equation: EQU .theta.=0.5f.lambda./v (5)
where f is a frequency of the ultrasonic wave, .lambda. is a wavelength of light, and v is an acoustic velocity in the acoustic cell 213a.
When the frequency of the ultrasonic wave S changes at a high speed to vary the diffracted angle .theta. rapidly, the acoustic cell 213a simultaneously includes acoustic waves of a certain band of frequency. Since the incident beam L.sub.b is farther from the transducer 213b than the incident beam L.sub.a, the incident beam L.sub.b is diffracted at an acoustic frequency f.sub.b higher than an acoustic frequency f.sub.a at which the incident beam L.sub.a is diffracted. A diffracted angle .theta..sub.b of the outgoing beam L.sub.b1 is therefore larger than a diffracted angle .theta..sub.a of the outgoing beam L.sub.a1. That is, when the ultrasonic wave is swept from a lower frequency to a higher frequency, the AOD 213 functions as a concave lens. On the other hand, when the ultrasonic wave is swept from a higher frequency to a lower frequency, the AOD 213 functions as a convex lens. These effects are called cylindrical lensing.
A method of compensating the cylindrical lensing effect is disclosed in Japanese Patent Laying Open Gazette No.60-107828, for example. According to the method, as shown in FIG. 3B, a compensating lens 213c is placed at the image side of the AOD 213. The compensating lens 213c makes the diffracted angles .theta..sub.c of the outgoing beams L.sub.a1 and L.sub.b1 equal to each other.
Since a difference between the diffracted angles .theta..sub.a and .theta..sub.b is small, a focal length of the compensating lens should be accurately set to make these angles equal to each other. Since a fabrication tolerance of a focal length is usually about plus or minus 5%, the compensating lens of higher accuracy is expensive.
Incidentally, the number of resolvable spots scanned by an AOD depends on a diameter of a light beam in an optical system. It is therefore desired to increase a diameter of a light beam to increase the number of resolvable spots. In order to meet this requirement, a lens system for expanding a light beam is placed near an AOD. If the compensating lens for compensating the cylindrical lensing effect is further added, the number of lenses used in the optical system becomes fairly large; this makes alignment of the lenses difficult.